Null circuits



March 8, 1960 NULL CIRCUITS Filed D90. 15, 1955 J. F. M CLEAN WW W 3Sheets-Sheet 1 JIIIS Ii 6 FIG.I FIG 2 5; PRIOR ART 2 PRIOR ART Fl G. 3 6

PRIOR ART I-- I9 l3 IO I'ri mil I2 I; 2-oc( 9 l6 ll FIG. 5

l'" e 20 2| 5 2s 3%; .5 Q3 II II 24 w -o3 FIG. 7 W f 21 29 I FIG. 8PRIOR ART uvmvrm JOSEPH F. M0 CLEAN FIG. 9 AGENT March 8, 1960 J. F.MCLEAN 2,923,001

' NULL cmcurrs Filed Dec. 15, 1955 5 she ts sneet 2 POWER 69 SUPPLYIOOV.

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,2 INVENTOR.

' l JOSEPH F. MCLEAN F0 F FREQUENCY By AGENT March 8, 1960 J. F. MOCLEAN2,928,001

7 NULL CIRCUITS File d Dec. 15, 1955 s Sheets-Sheet 3 IN VEN TOR.

JOSEPH F. MQCLEAN AGENT United States Patent NULL CIRCUITS Joseph F.McClean, Goshen, N.Y; I

Application December 15, 1955, Serial No. 553,228

i 13 Claims. (Cl. 250 27) This invention relates to frequency selectivenetworks .and, in particular, to frequency selective networks employingactive circuit elements. Further, this invention is primarily concernedwith such active frequency selective networks as are characterized bytransmission throughout an entire range of frequencies, except for astop band of relatively small transmission, approaching zerotransmission.

Such networks are useful in a manner. comparable to certain passivefrequency selective networks of the socalled parallel-T, bridged-T,m-derived and like varieties and will be shown in'the description tofollow to offer advantages of an electrical and physical nature oversuch networks in various applications.

Networks offering relatively narrow regions of small transmission arenormally employed under circumstances demanding either rejection of asingle undesired compo nent frequency in a waveform, while passingothers ideally unattenuated, or for producing incidentalphase alteringeffects among the several components of a waveform. The latter use isimportant in so-called COI'I'CC.

tion networks for compensating collateral time delays normally occurringin transmission systems. Depending upon the application, thetransmission region, in the neighborhood of the minimum transmission, ornull,

frequency may be described as exhibiting gradual slope or steep slope ofthe curve depicting relative transmission as a function of frequency,the former being-analogous to a condition of low Q-a nd the latteranalogous to a condition of high Q in a simple tuned circuit. Simiglarly depending'upo'n application, transmission may be described infrequency regions relatively removed from the stop band as (l) uniform,both for frequencies above and frequencies below the stop band, (2)uniform with given gain or attenuation, on one side of the, stop banduniform with a different gain or attenuatio'non the other side of thestop band, (3) non-uniform with at tenuation a function of frequency onone sides of the stop band.

The networks to be described, which incorporate the principles of thisinvention, will be for conveniencerestricted to the classes (1) and (2)above. However, the principles of this inventionmay apply equally wellto networks of class (3.) or to other classes not confined to thesethree. 1

An object of this invention is to provide a filter network having meansfor independently controlling null frequency, cut-off frequency and theQ of the network.

filter apparatus of the null type composed of resistance sideor on both7 Patented Mar. 8, 1960 and but one type of reactance, incombinationwithactivecircuit elements.

Another particular object is toprovide a compact null circuit suitablefor use at low frequencies.

Still another object is to providea high Q filter cir-.

cuit.

Still another object of this invention is to provide a filter circuitutilizing resistance elements and physically small inductance elementstogether with active elements while achieving the effects commonlyattainable with but large inductances. j v

Still other objects and advantages will be in part apparent, and in partpointed out with particularity as the Figure 3 shows schematically ahigh-pass filter section, H

of the m-derived type.

Figure 4 is a schematic showing ofa form of active network embracing theprinciples of this invention.

Figure 5 is a schematic showing a second form of active network withinthe scope of this invention.

Figure 6 is a schematic showing third type of active network within thescope of this invention.

Figure 7 represents schematically one formof passive,

so-called bridged-T filter network.

Figure 8 is a diagram of one form of resistance-capacitance passivenetworkcornmonly known in the art as a parallel-T circuit.

Figure 9 is a diagram of still another active uetwork based upon theprinciples of this invention.

Figure lOand Figure 11 are schematic showings of alternative embodimentsof active networks based upon the principles of this invention.

Figure 12 is a specific embodiment of the generalized circuit of Figure4.

Figure 13 is a graphical representation of the filter characteristic ofthe filter of Figure 12 expressed as Attenuation vs. Frequency.

Figure 14 is a generalized schematic of a circuit corresponding toFigure 4 with inductances employed in place of the capacitances.

Figure 15 is a generalized schematic of a circuit corresponding toFigure 5 with inductances employed in place of the capacitances.

Figure 16 is a generalized schematic of a circuit corresponding toFigure 6 with inductances employed in place of the capacitances.

Figure 17 is a generalized schematic of a circuit correspending toFigure 9 with inductances employed in place of the capacitances.

Figure 18 is a generalized schematic-of a circuit corresponding toFigure 10 with inductanc'es employed in place of the capacitanees.

Figure 19 is a generalized schematic of a circuit corresponding toFigure 11 with inductances employed-in place of the capacitances.

For the purpose of defining the electrical properties of 3 ratio andwill be used consistently as so defined in the discussion to follow.

Generically, the passive networks, described herein solely as exhibitingcharacteristics desirable in filter practice of the former art, may beclassed in two' categories;

those employing freely the circuit elements of resistance, capacitanceand inductance and those restricted in their composition to resistanceand but one kind of reactance. In general, networks of the first, orunlimited, variety are distinguished in the practical sense by greatersharpness, or steepness of slope of the transfer ratio in the vicinityof the frequency of minimum transmission, while practical networks ofboth types offer in theory the possibility of a true zero oftransmission at the null. This latter property is not strictlyattainable in some unlimited net- Works due to the finite losses ofpractical inductances. For this reason, where, in the following complexvoltages transfer ratios are developed for such networks, the inductivelosses will be neglected and the resultant transmission will appear topermit such perfect nulls.

Referring now to Figure 1, the terminals 1 and 2 taken together compriseinput terminals of a network, in which the inductance t and thecapacitor are connected in parallel, one end of the combinationterminating in the terminal 1 and the second end connecting to aterminal 3 which constitutes one output terminal of the network. Theother output connection to the network is made to the common terminal 2and the network is completed by the resistor 6 connected between theterminals 2 and 3. This network will be recognized as a simple type ofantiresonant trap circuit and, if the losses inherent in the inductance4 are neglected, can be readily shown to exhibit the complex voltagetransfer ratio:

Where w represents the angular frequency, 1' is a symbol equal to /1,and p represents the product jw. In this case the co-efiicients of theseveral terms are given by:

In Figure 2 there is represented a form of so-called m-derived low-passfilter section in which 1 and 2 comprise input terminals, 3 and 2.comprise output terminals, an inductance 4, interconnects the terminals1 and 3, a capacitor 5, likewise interconnects the terminals 1 and 3,the capacitor 7, is connected between the terminals 3 and 2 and aresistance 6 is also connected between the terminals 3 and 2, completingthe network. Once again neglecting the losses inherent in the inductance4, the complex voltage transfer ratio may be derived:

the symbols j and w respectively being /1 and the angular frequencyof'the applied wave and the symbol p again representing their product.The co-eflicients, in terms of the constants of the network, being Asimilar network is set forth in Figure 3 which represents a high passfilter section of the m-derived type, and in which 1 and 3 compriserespectively input and output terminals. The terminal 2 serves as acommon connection for both input and output. The inductance is connectedbetween the terminals 1 and 3, The capacitor 5 is likewise connectedbetween the terminals 1 and 3. The inductance f5 interconnects theterminals 3 and 2 which are also interconnected by the resistance 6which completes the network. The complex voltage transfer ratio for thenetwork of Figure 3 is found to be:

With 1', w and p defined as before, the several co-efficients for thecircuit of Figure 3 become:

ent common passive combinations of the three circuit parameters ofresistance, inductance and capacitance which have in their practicalembodiments useful electrical properties. Although the ideal performancepermitted by their respective complex voltage transfer ratios is notpossible with physically realizable inductances, in many applicationslosses of the reactive elements can be curtailed to a degree permittinga close approximation of the ideal. v In other applications, however, itis not possible to achieve sufi'iciently good components to produce theperformance desired. For example, when, in the course of designing suchpassive networks for operation at lower and lower frequencies, a pointis reached where inductors of reasonable size, Q and cost becomedifficult to produce, the passive networks discussed, as well as thoseoffering similar transfer ratios, do not provide a solution. It is thusan object of this invention to provide alternative networks, composed ofresistance and but one type of reactance, in conjunction with activecircuit elements, which not only overcome the practical shortcomings ofpassive networks, containing resistance and both types of reactance, butalso to provide such active networks capable of the theoreticalinfinite-Q performance of non-realizable passive networks.

Figure 4 is one form of active network embodying the principles of thisinvention, in which: the terminal 1 provides a means for connecting asource of input voltage, in association with the common terminal 2; theterminal 3 in association with the common terminal 2 forms a means forobserving the output voltage. The elements 9, 10, 11, 12 and 19'havingthe respective gain constants k k k k k are active unilateraltranslating devices whose common characteristic is their ability torepeat at their output terminals voltages applied to their respectiveinput terminals such voltages modified only by the said dimensionlessproportionality constants k k k k k and exhibiting the dual propertiesof relatively high input impedance and relatively low output impedancewith respect to theimpedances coupled to their terminals, as observed inthe vicinity of the so-called cut-off frequency of the entire network.The active devices 9 and 19 are mutually fed from the input terminal 1.The resistance 13 is connected between the output of the device 19 andthe input of the device 10. The capacitor =16 is similarly connectedfrom the output of the device .9 to the input of the device 11. Acapacitor 14 is connected to the input of the device '10 and theremaining terminal of the capacitor 14 is joined to the resistance 15,the remaining terminal of the latter being connected to the input sideof the device 11. A resistance 17 connects from the output side of thedevice 10 to the input side of the device 12 which is also connected toone end of .the capacitor 18. The opposite terminal of the capacitorlAnalysisof the circuit, of FigureAfor the, complex voltagetransferratio reveals the expression: T r

and upon the fulfilhnent of the condition:

R c, =.R,c,='Rc

the above-reduces to the. form:

E 'E+BP+DP the symbols jand w defined 'as before, the severalcoefficients become:

Thus, by proper selection of the circuit parameters R,

' lossless reactances was requiredto produce ;a perfect null in theexamples given, no-such restriction :exists for the embodiment of thisinvention in Figure 4, provided only that the condition R CiR C statedabove is fulfilled.

Figure is another embodiment of the principles of this invention whosecircuit is in all respects vsave one identical to that of the circuitof'Figure 4 and whose component parts bear identicalinumbers. Whereas inthecircuit of Figure 4. the capacitor 14 .is terminated at the outputconnection of the active device 12 in the circuit of Figure 5 thecapacitor ,14 is terminated instead at :the common circuit terminal 2,The form of the transfer equation is unchanged and the co-efiicientsofthe several terms are unchanged, with the exceptionof theseconddenominator'term co-eificient, which becomes:

Figure 6 is a representation of still another "circuit employing theprinciples of this invention, identical in all respects save one to thecircuit of Figure 4 and whose component parts are identical and bearidentical numbers in the drawing as those comprising the circuit ofFigure 4. Whereas in the circuit of Figure 4 the resist: ance 15 isterminated at one end at the output connection of the device 12 in thevcircuit of Figure 6 the resistance15- is connected instead to thecommon circuit terminal 2. The form of the transfer equation given forthe circuit of Figure 4 applies equally to the'equation for the complexvoltage transfer ratio of the circuit of Figure 6 and the co-efficientsof the terms, except for the co-eificient of the second denominatortermsare'identi cal. The second denominator terms co-efilcient is foundto be:

In practice the circuits of the several drawings, Figure 4, Figure 5 andFigure 6 difier in performance only'in the conceivable minimum value ofthe co-efiicient B. vAn examination of the mathematical effects ofcontrol over the magnitude of the said co-efiicient B reveals that thisco-efiicient controls the steepness of the plottedivoltage transferratio curve in the vicinity of the null, i.e., the sharpness of'the'null, such eifect appearingininverse of the co-efficient, B.

The usefulness of the described circuits of Figure 4,

'Figure'S and Figure 6 is not limited to the stimulation of drawings ofFigure .1, Figure 2 and Figure 3 which, it

has been noted, are capable of true nulls only when theoreticallycomposed of lossless reactances in whole or in part. Such circuitsembracing the principles of this invention may also be employed in lieuof those types of passive circuits which, though constituted ofrealizable reactances and their accompanying losses or resistivecomponents, are, by virtue of special design characteristics capable ofproducing true nulls. .An e'xampleof a class of networks capable of truenulls is the so-called bridged-T network of Figure 7.

Referring to Figure 7,'the terminals 1 and 2 constitute input'terminalsto a network, the terminal 2 being common .to both input and output. Theoutput terminals 3 and 2 provide access to the output voltage, e whichappears when the driving,.or input, voltage is applied to the inputterminals. The inductance 20 is connected between the terminals 1 andStogether with itsloss re-' sistance 21 here representedas a physicalresistance in series with the said inductance 20. The capacitor 22 isconnected at its one end to the terminal 1 and at its second terminus tothe capacitor 23 and to the resistance 24. The remaining terminal of thecapacitor 23 is connected to the circuit output terminal 3 and thecircuit is completed by'the connectionof the resistance 24 to the commonterminal 2.

The solution for the complex voltage transfer ratio of the circuit ofFigure 7 is of the form:

, where the symbols 1' and w represent-V-l and the angular frequencyrespectively, and the several co-efiicients are as follows:

Thus it is seen that the circuits of Figure 4, Figure 5 and Figure 6comprised of active circuit elements, re

sistance and but one itype'of reactance, may also be given performancecharacteristics similar to passive circuits, employing resistance andboth types of reactance known to the art asbridged-T circuits, with theadditional property, as respects the active network, of simple controlof circuit Q and the possibility of realizing cir- :,cuit Qs in excessofithe capabilities of the passive networks.

Referring now to Figure 8, the drawing illustrates a common type of nullcircuit of the prior art, known as a parallel-T. Again the network isprovided in the drawing with input terminals 1 and 2 of which 2 is aterminal common. to both input and output and with output terminals 3and 2. The resistance 25 is connected to the input terminal 1 at its onejuncture and to the capacitor connected at its far end to thecommon'circuit 2. The

circuit is completed by the junction of the remaining ends of theresistance (Aland the capacitor 29 to each other and to the outputterminal 3. Itis found upon ere analysis that the complex voltagetransfer ratio for this circuit is given by the equation:

Z 1 +1 2 2) 2102 (CZZRIRZ) -j 1 1 CZFRZ) r '8' 55' and capacitor 56.,The voltage appearing at the common junction of the two components55""and 56' is which equation reveals that the parallel-T networkexhibits a real null when the real and imaginary parts of the numeratorrespectively are zero, a condition requiring that R1C1=R2C2.

In Figure 9 is shown a network within the scope of this invention,which, while resembling the parallel-T in many respects, possessescharacteristics which render it markedly difierent and superior inperformance and utility to that of the said parallel-T circuit. Circuitele ments in the Figure 9 bear like numbers as those in the Figure 8 andare connected in like manner with the following exceptions: the additionof an active unilateral translating device 31, which has the attributesof such devices as previously described herein, so that its input isconnected to the circuit output terminal 3 and its output terminal formsa common junction with the capacitor 26 and the resistance 28 saidterminals of the said capacitor 26 and resistance 28 being thoseterminals connected in the circuit of Figure 8 to the common ter minal2.

Analysis of the circuit of Figure 9 reveals a complex voltage transferratio:

in turn applied tothe grid of cathode follower tube 57.,

The capacitor 64-resistor 69 arm of the network corresponding toresistor 15 and capacitor 16 is driven in like fashion with the voltageacross resistor 63. The junction of capacitor 64 andresistor 69is-connected to the grid of cathode follower tube 61.

A resistance 5Q-capacitance 60 branch is connected between therespective cathodes of cathode follower tubes 57 and 61, the junction ofresistor 59 and capacitor 60 being connected to the grid of cathodefollower tube 70, whose cathode provides output voltage through the"D.C.blocking capacitor 67. The output voltage may be derived from outputterminals 72. The grid of cathode follower tube 66 is connected to thevariable tap of resistor 71 permitting a selected portion of the outputvoltage to be applied between the junction of capacitor 56 and resistor59. The generalized circuit of Figure 4 terminates at terminal 3 whichpoint is also shown in Figure 12.

In operation the three R-C time constants are made equal (R C =R C =R C),where R is the resistance in ohms and C is the capacitance in farads.The

where the symbol k is a constant representing the gain of the activedevice 31 and the symbols 1', w and p are as previously defined. Thevnumerator of the latter expression is identical in all manner with thatof the transfer equation of the parallel-T circuit of Figure 8 thus theconditions necessary for a transmission null are also identical. it willbe noted that the interposition of the active device 31 serves to modifythe co-efiicient of the first and second order denominator terms in wand 12, such that increasing k serves to reduce the magnitude of theseco-eflicients, having the physical significance of increasing thesharpness or rapidity of change of transmission in the vicinity of thenull.

Figure 10 and Figure ll are modifications of the circuit of Figure 9 ina manner analogous to the relationship of the circuits of Figure 6 andFigure 5 to the circuit of Figure 4. Analysis of the circuits of Figure10 and Figure 11 reveal transfer equations identical in form to thetransfer equation applying to the circuit of Figure 9 and-differing fromit only as respects the interrelation between the active gain factor kand the co-eflicients of the first and second order denominator terms.

While I have elected'to show generalized schematic circuits so as tomake the basic concepts of this invention readily understood, I wish itto be appreciated that the practical embodiments corresponding theretomay be built in accordance with standard engineering practice. By Way ofexample, the circuit of Figure 4 has been implemented in Figure 12. Theother generalized circuits may be translated in a like manner. It shouldbe noted that while I have chosen to show triode vacuum tubes as thetranslating device, I regard the use of other translation devices withinthe scope of this invention.

Referring now to Figure 12, a pair of input terminals 50 have connectedthereto a D.-C. blocking capacitor 51, which is in turn connected to thegrid of cathode follower tube 53; this corresponds to terminal 1 ofFigure 4. Resistor 52 serves as a grid resistor and potentiometerresistor 54 the cathode resistor. This cathode follower stage serves asa matching and isolating stage and repeats across low impedance resistor54, the A.-C. signal, applied to high impedance input terminals 50.

A second cathode follower stage utilizes tube 68 driven from a variabletap of potentiometer 54. The resistor 13 and capacitor 14 of Figure 4are shown as resistor all) value of M is controlled by thertap settingof potentiometer 54- and the Q is adjusted by means of the tap,

setting. of potentiometer 71.

In practice particular filter characteristics are realized by thecascading of so-called filter sections, each ,of

which contributes important properties to the aggregate performance.Thus, by way of example, it is. common to compose a filter of one ormore sections of the socalled constant-k or similar typesuch sectionscon tributing continuingly greater attenuation with frequency removalfrom the pa-ssband-combined with one or'more sections of m-derived orrelated types; The said inderived sections contribute the desiredproperty of rapid attenuation beyond cut-0d in the region offrequenciesv In preparing sections of the constant-k or like type, con-'trol is ordinarily exercised over the cut-off frequency. the rate ofattenution per octave of frequency departure in the stop band and thepeaking, or Q factor, in the immediate vicinity of cut-off. Innz-derived or like sections control is available to the designer over.the' above properties and, in addition, over the null, or maximumattenuation, frequency.

Referring in; particular to the'network of Figure 4. and.

the corresponding general expression for the complex voltage transferratio thereof and also to the passive.

networks of Figure 2 and Figure 3 and the respective expressions fortheir complex voltage transfer ratios, it is apparent that saidexpressions are all identical as respects the frequency-containing termsand differ only as respects the several dimensionless co-efficients.Thus, specifically, to simulate entirely the voltage transfer-ratio ofthe low-pass network of Figure 2 by means of the network of the presentinvention shown in FigureA, one

need adjust the resistances, ,capacitances and gain con,

stants of thelatter to produce. co-e'fficients A, BQD, E

and G identical to those of the networkof Figure 2 maybe convenientlyadjusted, for example, by manipulation of the respective gain constantsk and k for simulation of circuit m. Further inspection will reveal thatthe conventionally defined cut-off frequency of the simulating circuitis controlled entirely by the coefficients D and 'E, which in turn areconveniently adjustable by manipulation of respective resistances andcapacitances of the network. Lastly, the peaking or Ql factor of thesimulation is tied to duplication of the co-eflicient B, which may beadjusted, without affecting prior adjusted cut-01f and null, bymanipulation of the gain constant k In an exactly parallel procedure itmay be shown that complete simulation of thepassive high-pass network ofFigure 3 may be had also by proper control of the design constants ofthe network of Figure 4 of. this invention. 7 a

By exactly similar procedures it may be shown that i the networks ofthis invention, Figure andqFigure 6,

may be employed to exactly simulate in respect to complex voltagetransfer ratio the passive networks described by Figure 2 and Figure 3as well .as those of Figure 1, Figure 7, Figure 8 and other networks.

When employed in lieu of the cited conventional passive networks andothers generically similar, certain advantages accrue which make suchsubstitution particularly advantageous: (l) The restriction of circuitelements to resistance-capacitance (or resistance-inductance)combinations and the corollary elimination of one form of reactance makepossible physical realization of properties not possible withconventional networks. Example: complete realization of low-frequencyfilter designs normally prohibitive or impossible to achieve usinginductances; (2) independent control over the separate properties ofnull frequency, cut-oif frequency and peaking (Q) not possible becauseof the physical interdependence of critical network properties in thelatter. Example: the peaking of conventional passive filters isinextricable tied, in some cases, to the physical limitations of Q ininductors, limiting the possibility of modifying this important propertyin the finished filter; (3)

cuit;.a first two terminal-resistor having one of said terminals'connected in series with said first translation device output circuit; asecond translation device having a high impedance input circuit and alow impedance output circuit, said input circuit being connected to theother of said terminals of said first resistor; a second two-tenterminal of said first reactance and said input circuit being connectedin parallel with said first translation device input circuit; a fourthtranslation device having a I terminals of said second resistor; a thirdtwo-terminal high impedance input circuit and a low impedance outputcircuit, said input circuit being connected .to the other terminal ofsaid first reactance; a second two-terminal reactance having one of saidterminals connected to said fourth translation device output circuit andthe other of said terminals connected tothe other of said connected tothe other said terminal of said third reactance; and means for feedingback a portion of the output signal to a portion of said apparatusisolated from said input signal introducing means by one of saidtranslation devices.

2. Theapparatus of claim 7 wherein said reactances are capacitors. I Y

3. A frequency sensitive apparatus comprising in com- Q bination: afirst translation device having a high impedance input circuit and a lowimpedance output circuit; a first two-terminal resistor having one ofsaid terminals connected in series with said first translation deviceoutput circuit; a second translation device having a separation of theproblem of achieving a suitable overall voltage transfer ratio in asection or in an entire filter from impedance conditions, either atinput or output, or at intermediate points among said sections. Designof passive resistance-inductance-capacitance filters is not possiblewithout close simultaneous attention'to'impedances as well as to voltagetransfer. Impedance considerations frequently impose severe limitationsupon the attainable voltage transfer properties.

A typical attenuation curve with reference to frequency is shown inFigure 13.

In Figures 14 to 19 I have shown inductances substituted for thecapacitances of the equivalent filters of Figures 4, 5, 6, 8, 9, 1i) andll.

In order to make readily apparent the substituted components, they havebeen identified as inductance 26L in place of capacitor 26 andinductance 27L in place of capacitor 27, etc.

While I have disclosed what is at present considered the bmt mode forcarrying out the invention, it will be obvious to those skilled in theart'that various changes and modifications may be made therein withoutdeparting from the invention, and it is, therefore, aimed in theappended claims to cover all such changes and modificahigh impedanceinput circuit and a low impedance output circuit, said input beingconnected to the other of said terminals of'said first resistor; asecond two-terminal resistor having one of said terminals connectedtosaid second translating device output circuit; a first two-ter- Iminal reactance; a third translation device having a high impedanceinput circuit and a low impedance output'circuit, said output circuitbeing connected to one terminal of said first reactance and said inputcircuit being connected in parallel with said first translation deviceinput circuit; a fourth translation device having a high impedance inputcircuit and a low impedance output circuit, said input circuit beingconnected to the other terminal of said first reactance; a secondtwo-terminal reactance having one of said terminals connected to said afourth translation device output circuit and the other of said terminalsconnected to the other of said terminals of said second resistor; athird two-terminal reactance having one terminal connected to the commonjunction of said first resistor and said second translation device inputcircuit; .a third two-terminal resistance having one terminalconnectedtothe common junction of said first reactance and said fourth translatingdevice input cirtions as fall within the true spirit and scope of theinpedance input circuit and a'low impedance output circuit; a fifthtranslation device having a high impedance input circuit and alowimpedance output circuit, said input circuit being connected to thecommon junction of said second resistor and said second reactance, saidoutput circuit being'connected so as to provide a signal to at least oneof said other terminals of said third reactance and said third resistorassociated frespectively with said second and fourth translation deviceinput circuits; means for introducing a signal to said first'and thirdtranslation device input circuits and means for deriving a signal fromsaid apparatus. j

4. .A frequency sensitive apparatus comprising-in combination: a firsttranslation device having a high impedance input circuit and alow-impedance. output cir- 7 11 cuit; a first two-terminal resistorhaving one of said terminals connected in series with said firsttranslation device output circuit; a second translation device having ahigh impedance input circuit and a low impedance output circuit, saidinput being connected to the other of said terminals of said firstresistor; a second two-terminal resistor having one of said terminalsconnected to said second translating device output circuit; a firsttwo-terminal reactance; a third translation device having a highimpedance input circuit and a low impedance output circuit, said outputcircuit being connected to one terminal of said first reactance and saidinput circuit being connected in parallel with said first translationdevice input circuit; a fourth translation device having a highimpedance input circuit and a low impedance output circuit, said inputcircuit being connected to the other terminal of said first reactance; asecond two-terminal reactance having one of said terminals connected tosaid fourth translation device output circuit and the other of saidterminals connected to the other of said terminals of said secondresistor; a third two-terminal reactance having one of said terminalsconected to the common junction of said first resistor and said secondtranslation device input circuit and the other of said terminalscomprising a common input-output terminal for said apparatus; a thirdtwo-terminal resistor having one terminal connected to said fourthtranslation device input circuit; fifth translation device having a highimpedance input circuit and a low impedance output circuit, said inputcircuit being connected to the common junction of said second resistorand said second reactance and said output circuit connected to the othersaid terminal of said third resistor; means for introducing a signal tosaid first and third translation device input circuits and means forderiving an electrical signal from said apparatus.

5. A frequency sensitive apparatus comprising in combination: a firsttranslation device having a high impedance input circuit and a lowimpedance output circuit; a first two-terminal resistor having one ofsaid terminals connected in series with said first translation deviceoutput circuit; a second translation device having a high impedanceinput circuit and a low impedance output circuit, said input circuitbeing connected to the other of said terminals of said first resistor; asecond two-terminal resistor having one of said terminals connected tosaid second translating device output circuit; a first twoterminalreactance; a third translation device having a high impedance inputcircuit and a low impedance output circuit, said output circuit beingconnected to one terminal of said first 'reactance and said inputcircuit being connected in parallel with said first translation deviceinput circuit; a fourth translation device having a high impedance inputcircuit and a low impedance output circuit, said input circuit beingconnected to the other terminal of said first reactance; a secondtwo-terminal reactance having one of said terminals connected to saidfourth translation device output circuit, and the other of saidterminals connected to the other of said terminals of said secondresistor; a third two-terminal reactance having one terminal connectedto the common junction of said first resistor and said secondtranslation device input circuit; a third two terminal resistance havingone terminal connected to said fourth translation device input circuitand said other ofsaid terminals comprising a common input-outputterminal of said apparatus; a fifth translation device having a highimpedance input circuit and a low impedance output circuit, said inputcircuit being connected to the common junction of said second i stor andsaid second reactance, said output circuit being connected to the otherof said terminals of said third reactance; means for introducing anelectrical signal to said first and third translation device inputcircuits and means for deriving an electrical signal from aid apparatus.

6. A' frequency sensitive apparatus 'for producing an output signaldifiering from an input signal including signal input means and meansfor deriving an output signal comprising: a first branch circuit,interconnecting said signal input means and said signal output means,including at least one translation device having a high input circuitimpedance and a low output circuit impedance and a plurality ofresistors in cascade; a second branch circuit in parallel with saidfirst branch circuit comprising at least one translation device having ahigh impedance input and low impedance output circuit and a plurality ofreactances in cascade; and a feedback circuit including a translationdevice, having a high input circuit impedance and a low output circuitimpedance in cascade with an impedance element, said feedback circuitbeing interposed between said means for deriving an output signal and anintermediate portion of one of said branch circuits.

7. A frequency sensitive apparatus for producing an output signaldiffering from an input signal, including a two terminal signal inputcircuit means, including a first terminal and a second terminal, and atwo terminal signal output circuit means, including a third terminal andsaid second terminal, comprising: two branch circuits, one of saidbranch circuits including a plurality of resistor elements connected incascade between said first and'third terminals and the other of saidbranch circuits including a plurality of reactance elements con nectedin cascade between said first and third terminals so that said branchcircuits are in parallel; a signal feedback circuit connected betweensaid third terminal and a point on one of said branch circuits between apair of said cascaded elements of said branch circuit, said signalfeedback circuit including an element of the type included in theother'of said branch circuits anda translation device having a highimpedance'input circuit and a low impedance output circuit, said lastnamed element being connected in cascade between said one branch circuitand said translation device output circuit; a signal coupling circuitincluding said translation device connected between said signal outputmeans, and a point on said other branch circuit between a pair of saidcascaded elements of said other'branch circuit; and an element of thetype included in the said one branch circuit connected in series withsaid signal coupling circuit.

8. The apparatus of claim 7 wherein translating devices are interposedbetween each of said branches, and said signal input means, saidtranslating devices being characterized by a high impedance inputcircuit and a low impedance output circuit.

9. The apparatus of claim 7 wherein translating devices having highinput impedance circuits and low impedance output circuits areinterposed between said branch circuits and said signal input means andbetween succeeding cascaded elements in each of said branch circuits.

10. A frequency sensitive apparatus for producing an output signaldiffering from an input signal includ ing signal input means and meansfor deriving an output signal comprising: a first branch circuit,interconnecting said signal input means and said signal output means,including at least one translation device, having a high input circuitimpedance and a low output circuit impedance, and a plurality ofresistors in cascade; a second branch circuit in parallel with saidfirst branch circuit comprising at least one translation device, havinga high impedance input and a low impedance output circuit, and aplurality of reactances in cascade; and a feedback circuit comprising atranslation device having a high impedance input circuit and a lowimpedance output circuit in cascade with an impedance element, saidfeedback circuit being interposed between said means for deriving anoutput signal and a point common to a plurality of said reactances insaid second branch circuit so termediate portion of one of said branchcircuits.

11. A frequency sensitive apparatu's for producing an output signaldiffering from an input signal including signal input means and meansfor deriving an output signal comprising; arfirst branch circuitinterconnecting said signal input means and said signal output meansincluding at least one translation device having a high input circuitimpedance and a low output circuit impedance and ajplurality ofresistors in cascade; a second branch circuit in parallel withsaid firstbranch circuit comprising at leastone translationdevice having a high,

impedance inputiand low impedance output circuit and .f y

a plurality of reactances in cascade; and a feedback circuit comprisinga translation device having a high impedance input circuit anda lowimpedance output cir-' cuit in cascade; with an impedance element, saidfeedback circuit being interposed between said means for deriving anoutput signal and appoint common to a plurality of said resistors insaid first branch circuitso as'to feed back a portion of said outputsignal to an inter-- mediate portion of one of said branch circuits.

1 :14 a said meansjfior deriving an output signal and an intermediateportion of one of'said branch circuits.

13. A frequency sensitive apparatus for producing an output signaldiffering from an input signal including signal input" means and, meansfor deriving angoutput signal comprising: a first branch circuitinterconnecting said signal input means and said signal output meansincluding at least one translation device, having a high input circuitimpedance and a low output circuit imedance anda plurality ofresistorsin cascade; a second branch circuit in parallel with said first branchcircuit comprising at least one translation-device, having a highimpedance input and low impedance output circuit, and

a plurality of reactances in cascade; and a feedback cir- 12. Afrequency sensitive apparatus for producing an device having a highimpedance input circuit and alow impedance output circuit in cascadewith an impedance element, said feedback circuit being interposedbetween r having a high impedance input-circuit and a low i111 pedanceoutput circuit interposed betweenza pair of said reactances; and afeedback circuit including a translation cuit comprising a two-terminalreactance, a' two-terminal resistance and a translation device having ahigh im-' pedance input circuit and a low impedance output circuit, saidinput circuit being connected to said means for'deriving an outputsignal and said output circuit connected to one terminal of eachof saidtwo-terminal resistance and said two-terminal reactance,the other saidterminal of said resistance being connected to a pointcommon to aplurality of said reactances and the other said terminalof-said'reactance being connected to a point common to a plurality ofsaid resistancesfor feeding back a portion of said output signal "tointermediate portions of said branch circuits.

References Cited in the file ofthis patent UNITED STATES PATENTS 3 2,341,067 Wise Feb. 8, 1944 2,565,497 'Harling .1. Aug. 28, 1

. 2,581,456, Swift Ian. 8, 1952 2,658,993 Seeley Nov. 10, 1953 2,672,529Villard Mar. '16, 1954 2,692,343 Spiro --Oct. 19, 1954 2,756,283 BachJuly 24, 1956 Sziklai. r Nov. 20, 1956'

